Constant potential output device having low temperature coefficient and overload protection



Nov. 24, 1970 ATURE F. KRAUSSER CONSTANT POTENTIAL OUTPUT DEVICE HAVING LOW TEMPER COEFFICIENT AND OVERLOAD PROTECTION Filed Feb. 19, 1968 mumDOm U0 8 R w ww INVENTOR. FRIEDRICH JOHANN KRAUSSER iw a Q ATTORNEY United States Patent O 3,543,140 CONSTANT POTENTIAL OUTPUT DEVICE HAV- ING LOW TEMPERATURE COEFFICIENT AND OVERLOAD PROTECTION Friedrich Johann Krausser, Jericho, N.Y., assignor to Emerson Electric Co., St. Louis, Mo, a corporation of Missouri Filed Feb. 19, 1968, Ser. No. 706,536 Int. Cl. G05t N58 US. Cl. 32322 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to a regulated power supply or voltage regulator circuit, and, more particularly, relates to a solid-state power supply which is immune to ambient temperature variations.

One major disadvantage associated with the use of solid-state components resides in the fact that such devices are extremely sensitive to ambient temperature changes. For most applications this characteristic may not be a design criterion; however, in those circuits requiring critical values of current or potential, temperature compensation, it becomes of paramount importance.

For example, regulated power supplies or voltage regulator circuits must maintain, of necessity, a constant potential difference between their output terminals. In solidstate power supplies or power supplies utilizing semiconductor components, ambient temperature variations cause corresponding variations in the characteristics of the components, thereby producing voltage variations at the output terminals. Hence, such supplies are rendered unfit for their intended use. While many temperature compensation circuits have been proposed to ameliorate this situation, such compensation circuits conventionally utilize a great many other components. Hence, such devices have been found to be uneconomic in terms of both cost and space.

Accordingly, an object of the present invention is to provide a regulated power supply which is independent of ambient temperature variations.

Another object of this invention is to provide a regulated power supply or voltage regulator circuit which is simple in construction and reliable in operation.

A further object and feature of the present invention resides in the novel details of construction which provide a regulated power supply of the type described which is economic to fabricate, and which occupies a relatively small volume.

A further disadvantage of solid-state devices is their susceptibility to burn out owing to overloads during even relatively small time intervals.

Hence, a further object of this invention is the provision of a regulated power supply which automatically turns off in the event of overload, as, for instance, when the output terminals are shorted.

Accordingly, a regulated power supply constructed in accordance with the present invention comprises a pair of input terminals adapted to be connected with a DC. source of potential, and a pair of output terminals adapted to be connected across a load. A substantially constant voltage device having a first temperature coeflicient, and an impedance means having a temperature coefiicient equal in magnitude to said first temperature coefi'icient, but opposite in sense, are connected in series between the pair of output terminals. A current regulating means is connected between said pairs of input and output terminals for supplying a constant current to said series circuit to maintain the potential between said pair of output terminals at a constant predetermined level. Ambient temperature changes cause equal and opposed voltage variations across the constant voltage device and the impedance means which cancel the effect of each other. Hence, the potential between the pair of output terminals remains at the predetermined level.

Other objects of the invention will be set forth hereinafter, or will be apparent from the description and the drawings, in which is illustrated an embodiment exemplifying the invention.

The invention, however, is not intended to be restricted to any particular construction, or any particular arrangement of parts, or any particular application of any such construction or arrangement of parts, or any specific method of operation or use, or any of the various details thereof, even where specifically shown and described herein, as the same may be modified in various particulars, or may be applied in many varied relations, without departing from the spirit and scopeof the invention, of which the exemplifying embodiment, herein shown and described, is intended only to be illustrative, and only for the purpose of complying with the requirements of the Statutes for disclosure of an operative embodiment, but not to show all the various forms and modificationsin which the invention might be embodied.

On the drawings, in which the same reference characters refer to the same parts throughout, and in which is disclosed such a practical construction, the single figure is a schematic circuit wiring diagram of a regulated power supply, as an exemplification of the present invention.

Referring to the figure, there is shown one embodiment of a regulated power supply or voltage regulator circuit, designated generally by the reference numeral 10, as an illuustration of the present invention. Supply 10 includes a pair of output terminals 12 and 14, which are adapted to be connected with a load (not shown). Connected between terminals 12 and 14 by a lead 16 is a series circuit 17 comprising a back-biased Zener diode or constant voltage device 18, and a negative temperature coefficient impedance or resistor 20, such as a thermistor or the like. Terminal 14 is further connected to ground by a lead 22. Connected in parallel with series circuit 17, comprising diode 18 and resistor 20, is a constant current source, designated generally by the reference numeral 24 (shown enclosed by the dash lines) which supplies a constant current to series circuit 17.

The potential appearing between output terminals 12 and 14 will be equal to the sum of the potential drops across resistor 20 and diode 18. While the voltage drop across diode 18 is substantially independent of the current flowing therethrough in the Zener region, the voltage drop across resistor 20 is directly proportional to the current flowing through it. Hence, for a constant current flowing through series circuit 17, the potential drop across this circuit and, therefore, the potential between output terminals 12 and 14 similarly will be constant.

It is a known fact that Zener diodes above a preselected voltage level have a positive temperature coefficient so that the voltage drop across such devices increases with a rise in temperature. However, in accordance with the present invention, resistor 20 is selected to have a temperature coefficient which is equal in magnitude but opposite in sense to the temperature coefficient of Zener diode 18. Hence, the variations in the voltage drops across the respective elements 18 and cancel each other so that the potential between output terminals 12 and 14 remains at the preselected constant level.

For example, if Zener diode 18 has a temperature coefficient of +20 mv./ C. at the operating potential, then a resistor 20 is selected having a temperature coeflicient of 20 mv./ C. at the particular value of current. If the ambient temperature rises 1 C., the voltage drop across diode 18 rises 20 mv., and the voltage drop across resistor 20 decreases 20 mv. Hence, the total change in potential across series circuit 17 and, therefore, appearing between output terminals 12 and 14, owing to temperature variations, is zero. Thus, regulated power supply 10 maintains a constant potential between the output terminals which is substantially immune to temperature variations.

Constant current source 24 includes a pair of input terminals 26 and 28 between which is connected a D.C. source of potential 30. Terminal 28 is connected to ground. Moreover, D.C. source 30 may comprise a battery or a rectified A.C. source of potential, or the like. Also connected between input terminals 26 and 28 and output terminals 12 and 14 is a current regulating circuit, designated generally by the reference numeral 32, which regulates the current flowing therethrough to supply the constant current to series circuit 17 and a load which may be connected between terminals 12 and 14.

The current regulating circuit 32 includes a serially connected PNP transistor or variable impedance 34 having an emitter or input electrode, a collector or output electrode, and a base or control electrode. The emitter electrode of transistor 34 is connected to terminal 26 through a resistor 36 by a lead 38. The collector electrode of transistor 34 is connected to output terminal 12 by a lead 40. Connected to transistor 34 is a negative feedback network 42 which maintains the current flowing through the collector electrode of transistor 34 at a constant value.

More specifically, the network 42 includes an NPN transistor or variable impedance 44 having electrodes similar to transistor 34. The base electrode of transistor 44 is connected to lead 40 by a lead 46. The collector electrode of transistor 44 is connected to the base electrode of transistor 34 and input terminal 26, through a resistor 48, by a lead 50. The emitter electrode of transistor 44 is connected to ground through serially connected resistors 52 and 54. Resistor 54 is shown connected between a pair of output terminals 56 and 58 since resistor 54 may comprise any load which requires a constant potential thereacross. In other words, a constant potential will appear across any impedance connected between terminals 56 and 58. Additionally, a resistor 60 is connected between the emitter and collector electrodes of transistor 44 for reasons which will become apparent from a consideration of the operation of the circuit.

If D.C. source 30 comprises an A.C. rectified source, it may be desirable to provide an additional degree of filtering. Accordingly, a capacitor 62 is connected in parallel with resistor 48, and a capacitor 64 is connected across resistor 54, to short A.C. hum signals to ground.

In operation, it is to be noted that the potential between the base electrode of transistor 44 and ground is constant since the potential therebetween is determined by series circuit 17. Resistor 60 is chosen to be willciently high so that it draws negligible current. Accordingly, the fixed base potential maintains the current flowing through the emitter-collector path of the transistor 44 at a constant level.

The current flowing through the collector electrode of transistor 44 is equal to the sum of the base current of resistor 34 and the current flowing through resistor 48. Since this base current is negligible compared to the current flowing through resistor 48, it can be assumed that transistor 44 maintains the current through resistor 48 at a constant value. Hence, the voltage drop across resistor 48 is maintained at a constant level.

However, resistor 48 is connected in parallel with the series circuit of resistor 36 and the base-emitter junction of transistor 34. Thus, the voltage drop across this series circuit similarly is maintained at the constant level, thereby to maintain the current flowing in the emittercollector path of transistor 34 at a constant magnitude. As a result, the current flowing through lead 40 (i.e., the collector electrode of transistor 34) is maintained at the constant level noted above.

It is to be noted that constant current source 24 automatically protects transistors 34 and 44 against overloads. For example, if a short circuit occurs between terminals 12 and 14, the current through series circuit 17 will decrease. Hence, the potential at the base electrode of transistor 44 similarly will decrease, thereby decreasing the current in the emitter-collector path of that transistor. As a result, the potential across resistor 48 will decrease, thereby causing the current flowing through the emitter-collector path of transistor 34 to drop. This action will continue until transistors 34 and 44 are driven into cut-off, thereby preventing damage to the transistors.

It will be obvious, from a consideration of the above circuit, that transistor 44 will not conduct initially. Hence, resistor 60 is included to cause a voltage drop to exist between the emitter and collector electrodes of transistor 44, to initiate conduction. Moreover, it is to be noted that transistors of opposite or complementary conductivities may be used for the respective transistors disclosed, with appropriate changes in the biasing arrangements.

Accordingly, a solid-state regulated power supply has been disclosed which produces a constant output voltage regardless of variations in ambient temperature.

What is claimed as new and useful is:

. 1. A regulated power supply comprising first and second lnput terminals adapted to be connected to a D.C. source of potential, first and second output terminals adapted to be connected with a load, a substantially constant voltage device having a first temperature coeflicient, impedance means having a temperature coefficient equal in magnitude to said first temperature coeflicient and opposite in sense, lead means for connecting said voltage device and said impedance means in a series circuit between said first and second output terminals, and current regulating means connected between said first and second input and output terminals for supplying a constant current to said series circuit to maintain the potential between said first and second output terminals at a constant predetermined level, said current regulating means including a first transistor having an emitter, a collector and a base electrode, a first resistor, lead means for connecting said first resistor between said first input terminal and the emitter electrode of said first transistor, lead means for connecting the collector electrode of said first transistor to said first output terminal, and feedback means connected between the collector and base electrodes of said first transistor for maintaining the current flowing through said collector electrode at a preselected substantially constant value, said feedback means comprising a second transistor having an emitter, a collector and a base electrode, lead means for connecting the emitter-collector path of said second transistor between said base electrode of said first transistor and said second input and output terminals, lead means connecting the base electrode of said second transistor directly to the connection of said first output terminal and said collector electrode of said first transistor, and a second resistor connected between the base electrode of said first transistor and said first input terminal, whereby said second transistor maintains a substantially constant cur- 6 rent flow through said second resistor, said first and sec- 3,094,654 6/1963 Roelli 323-4 X ond transistors having opposite conductivities. 3,293,540 12/1966 Silard et a1 323-68 2. A regulated power supply as in claim 1, in which 3,300,710 1/ 1967 Knauss 32317 said constant voltage device is a Zener diode having a 3,419,789 12/ 1968 Gately 323--22 positive temperature coefiicient, and said impedance means 5 is a resistive device having a negative temperature coefii- J D MILLER, Prlmary Examiner cient.

References Cited G. GOLDBERG, Asslstant Examiner UNITED STATES PATENTS us. (:1. X.R. 2,832,035 4/1958 Bruck et a1. 323-22 10 317 33;32368 3,069,617 12/1962 Mohler 32322 

